Step Feed 2 – 5 stage Anoxic Oxic Process

The original step feed process was introduced in the late 1930’s as a means of overcoming the perceived inefficiency in aeration of plug flow plants of the time. In principle the incoming influent flow was split into several streams (nominally roughly equal) and introduced into the aeration basin at approximately equal intervals.

The process evened out the aeration demand along the plant, but suffered from the potential of incomplete nitrification and “activation”, i.e. the removal of adsorbed carbonaceous substrates. It also introduced the possibility of excessive denitrification in the clarifiers with the resulting poor solids separation.

Correct implementation of step feed could overcome these possible problems by simply ensuring that the last addition of influent was far enough from the process outlet to provide sufficient aerobic retention to oxidize substrates held in the flocs.

The process had several subsequent rebirths, culminating in the present incarnation which makes use of several characteristics inherent in this plug flow process.

(1) Development of a series of anoxic zones from the influent introduction point, if no aeration is applied.

(2) Accumulation of solids at the head of the plant with a concurrent reduction of solids at the outlet, thus reducing clarifier loadings.

(4) Absence of the large nitrified liquor recycle (‘a’ recycle), which saves energy.

(5) Simple implementation in existing plants as a process upgrade.

(6) Efficient removal of both BOD and nitrogen.

(7) More control options and flexibility then available in basic plant configurations.

(8) Better accommodation of bulking sludges.

(9) The possibility of improving nitrogen removal using secondary anoxic zones and/or external carbon addition at the end of the process. Secondary anoxic zones are simply the provision of an extra pass with zero influent feed.

(10) The possibility of “swing” zones (zones with the capacity to operate anoxically or aerobically as required) using banks of aerators which may be operated independently of the majority of the aeration.

(11) Reduced aeration costs have been reported by some authors, but others suggest little in the way of savings. Conversely the system offers a more complex series of challenges to both designers and operators. Establishing the

operators to make use of changes on the fly during tuning of the process.

The variation of process solids from high at the head of the process to low at the end, makes control of sludge age more complex since the estimation of total process solids over a series of variably sized zones of different MLSS concentration is more difficult. Oxygen carry over from aerobic to anoxic zones is also a potential problem if a large number of passes are used.

The optimization of the process is hampered somewhat by the sensitivity of the system to basic parameters like TKN/COD ratio and the ‘s’ recycle. Estimation of F/M ratio is also more complex.

Process flexibility is enhanced by the capacity to split the feed into a wide range of different amounts, and similarly the zones or passes may also be sized differently along the plant. Anoxic and aerobic zones must be sized to suit the split entering the anoxic zone of the particular pass.

The normally accepted procedure is to size the first anoxic zone and the split to just denitrify the nitrate in the return sludge. From there each zone (anoxic and aerobic) should be sized to address the relevant load applied (i.e nitrate- N and ammonia-N respectively). The overall anoxic and aerobic mass fractions will still fall more or less into the accepted ranges of plants like the MLE.

Various split sizes and zone sizes have been used, and a selection of these are included below for 3 pass systems. However the optimums will depend on the influent character and effluent requirements.

Stages

1

2

3

4

5

% Feed split

33

40.5

26.5

% Anox/Ox

4/12.5

10.5/21.5

18/33.5

Notes: HRT 9.3 hours, SRT 11 days

Stages

1

2

3

4

5

% Feed split

27

38

35

% Anox/Ox

5/10

15/15

20/35

Notes: HRT 9 hours, SRT 11.3 days

Stages

1

2

3

4

5

% Feed split

38

38

24

% Anox/Ox

7

31

62

Stages

1

2

3

4

5

% Anox/Ox

5/10

10/20

15/40

It has been suggested that the optimum splits usually lie between 40:40:20 and 33:33:33 depending upon the influent characteristics and effluent requirements. The flow distribution determines the anoxic and aerobic volumes of the first two passes, while the final pass depends upon the total anoxic mass fraction required which is influent character and effluent requirement dependent also.

The process will still have the same robust operational character of nitrifying and denitrifying activated sludge processes, but this will depend somewhat upon the ultimate process design and in particular the capacity of the latter passes.

Literature reports have described 2, 3 and 4 pass systems which are favored variously in different countries, so the configuration here offers a maximum of 5 passes to accommodate all of these.

This process offers a more complex series of challenges to both designers and operators. Establishing the optimum process configuration is more difficult, and introducing process flexibility requires better training of process operators to make use of changes on the fly during tuning of the process.

The variation of process solids from high at the head of the process to low at the end, makes control of sludge age more complex since the estimation of total process solids over a series of variably sized zones of different MLSS concentration is more difficult.

Oxygen carry over from aerobic to anoxic zones is also a potential problem if a large number of passes are used, and needs to be monitored carefully.

The optimization of the process is hampered somewhat by the sensitivity of the system to basic parameters like TKN/COD ratio and the ‘s’ recycle. Estimation of F/M ratio is also more complex.

Process flexibility is enhanced by the capacity to split the feed into a wide range of different amounts, and similarly the zones or passes may also be sized differently along the plant. Anoxic and aerobic zone sizing must suit the split entering the anoxic zone of the particular pass.

The normally accepted procedure is to size the first anoxic zone and the split to just denitrify the nitrate in the return sludge. From there, each zone (anoxic and aerobic) should be sized to address the relevant load applied (i.e nitrate-

N and ammonia-N respectively). The overall anoxic and aerobic mass fractions will still fall more or less into the accepted ranges of plants like the MLE.

It has been suggested that the optimum splits usually lie between 40:40:20 and 33:33:33 depending upon the influent characteristics and effluent requirements. The flow distribution determines the anoxic and aerobic volumes of the first two passes, while the final pass depends upon the total anoxic mass fraction required which is influent character and effluent requirement dependent also.